1,720,980 research outputs found
Application of advanced transient laser polarimetry to study of nonlinear reflective polarisation phenomena in solids
No full textA novel femtosecond time-resolved polarimeter for reflective measurements was developed. The instrument was used with a picosecond Nd:YAG laser and an advanced femtosecond Ti:Sapphire laser. Sensitivity to pump-induced reflected probe polarisation rotation of better than 10-6 radians combined with time resolution up to 70 femtoseconds has been achieved. Time resolution of 30 femtoseconds was achievable with loss of sensitivity of approximately an order of magnitude. The polarimeter has been tested in a variety of spectroscopic configurations and with different types of solids (metals, semiconductors and superconductors) and was found to be a useful tool for the investigation of the transient nonlinear optical properties of opaque materials. A new type of optical autocorrelator for measurement of the duration of femtosecond optical pulses utilizing a reflective polarisation effect in metallic nickel was designed.The recently developed spectroscopic polarimetric technique of the Specular Inverse Faraday effect was for the first time used to study a semimagnetic semiconductor (Cd0.6Mn0.4Te), a high Tc superconductor (YBaCuO7-δ) and a ferromagnetic metal (Ni). The cubic optical nonlinearity (χ(3)1122 - χ(3)1221) was measured in these opaque solids for the first time. In particular, in nickel a strong (χ(3) ~ 2 x 10-9esu) and exceptionally fast (τ < 30 fs) nonlinearity, which is promising for reflective nonlinear optical applications, was found. The thermalisation and spin-relaxation dynamics in GaAs were investigated with femtosecond time resolution using Specular Inverse Faraday effect spectroscopy for the first time.Investigations were undertaken into natural and light induced reflective anisotropy in zincblende structure crystals. Transient measurements of induced anisotropy, its orientation dependence and intensity dependence were made for the first time in GaAs, InSb and Cd0.6Mn0.4Te. Analysis of microscopic and macroscopic pictures of the light-matter interaction in zincblende structure materials allowed attribution of the observed anisotropic effects to the phenomenon of natural and induced gyrotropic linear dichroism.A computer program based on the quaternion description of light polarisation, the constitutive equation and the wave equation and suitable for calculation of evolution of partially polarised light in linear and nonlinear optical media with local and nonlocal optical response has been developed. The evolution of the polarisation state of partially polarised light during propagation in an isotropic medium with cubic nonlinearity has been investigated in detail and compared with published analytical and semi-analytical data
Sub-microsecond pulsed pumping as a means of suppressing amplified spontaneous emission in tandem pumped fiber amplifiers - Data in the figures
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Overlapped pulsed pumping of tandem pumped fiber amplifiers to increase achievable pulse energy
Full text linkIt has been reported previously that in the regime appropriate for amplifying femtosecond pulses using the chirped pulse amplification technique in Yb-fiber sources that sub-micro-second pulsed tandem pumping not only provides the thermal benefits of c.w. tandem pumping but also enables strong suppression of ASE. In that case, the pump pulse preceded the signal pulse train. Here we propose a tandem pumping scheme in rare-earth-doped fiber amplifiers, where a train of signal pulses is amplified by a pump pulse which is almost exactly temporally overlapped. Simulations demonstrate that this can be used to create uniform gain across the signal pulse train, even at very high total pulse energies where there would be significant gain shaping in the previous case. In addition the pump is absorbed in a much shorter length, which increases the threshold for nonlinear effects and gain of greater than 26 dB is shown to be readily achievable in an amplifier as short as 1.5 m. This results in increased extractable energy before reaching the threshold for limiting nonlinear effects such as stimulated Raman scattering. These attributes should be attractive for high energy, high average power, ultrashort pulse, coherently combined fiber laser systems
Sub-microsecond pulsed pumping as a means of suppressing amplified spontaneous emission in tandem pumped fiber amplifiers
Full text linkWe propose a new tandem pumping scheme in three-level rare-earth doped optical fibers where energy storage in a pump fiber laser enables much shorter (~500 ns) pump pulses than is possible with pulsed diode laser pumping. This has the effect of reducing the time during which the final amplifier has high inversion and hence reduces the ASE when compared to continuous wave (c.w.) tandem pumping. We simulate the gain and ASE dynamics in a pulse-pumped Yb-doped fiber amplifier and show that the ratio of amplified signal to ASE background can be improved by up to 2 orders of magnitude over a large range of pulse energies compared to c.w. tandem pumping. Sub-micro-second pulsed tandem pumping offers a relatively straightforward, as yet experimentally untried, way to substantially reduce undesirable ASE compared to the levels in diode-pumped fiber amplifiers
Detailed comparison of injection-seeded and self-seeded performance of a gain-switched laser diode
No full textWe investigate the performance of a gain-switched picosecond laser diode operating at 1.06µm when either injection- or self- seeded. We conclude that the much simpler self-seeding arrangement leads to no degradation of performance
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Ultrashort pulse laser and amplifier systems based on Yb-doped holey fiber
No full textCombining an active Ytterbium doped core with the unusual nonlinear and dispersion properties of holey fiber provides opportunities for rapid development of a wide range of diode-pumped ultrashort pulse lasers and amplifiers. Moreover, using the holey structure to guide pump light creates a highly efficient cladding pumped fiber. We describe a number of ultrashort pulse applications based on this rapidly developing technology
Numerical optimisation of pump absorption in doped double-clad fiber with transverse and longitudinal perturbation
No full textWe report on the impact of the cross-sectional shape of the inner cladding of double-clad fiber on pump absorption using a Beam Propagation Method based on azimuthal harmonics expansions and the so-called "superformula" to describe complex shapes. The impact of shaped cladding and of the fiber layout on pump absorption of double-clad fiber is studied. We observe that for nearly circular cross-section fibers the pump absorption can be enhanced to match that of fiber with strong azimuthal asymmetry, through optimised fiber layout that maximizes pump mixing
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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